1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, and relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine.
2. Related Background Art
In an image forming apparatus which electrophotographically forms an image, the image density varies greatly depending on the environment where the image forming apparatus is placed (temperature, humidity, and the like), the durable time period, the photosensitive member, the variation in the characteristics of the developer, and the like. Particularly, in a color image forming apparatus, the hue or tone also varies.
Therefore, conventionally, a toner pattern for detecting the density (hereinafter referred to as a xe2x80x9cpatchxe2x80x9d) is formed from a photosensitive member onto an intermediate transfer member or a transfer material bearing member for bearing a transfer material. By carrying out a density correcting mode for detecting the density of the patch (hereinafter referred to as xe2x80x9cpatch detectionxe2x80x9d) using a density detecting sensor, conditions of the image forming process such as the charging bias, the developing bias, the exposure dose are controlled to make appropriate the image density.
Further, the density detecting sensor for detecting the patch density is, due to the limited space for attachment, attached to a position opposing to the intermediate transfer member or to the transfer material bearing member.
In an image forming apparatus for forming a full color toner image on the intermediate transfer member using four colors and four photosensitive members, when the above-described patch detection is carried out, a patch in a first color (for example, yellow) comes in contact with the three other photosensitive members after it is transferred from the photosensitive drum to the intermediate transfer member and before its patch density is detected by the density detecting sensor. Here, since the patch formed on the intermediate transfer member comes in contact with the other photosensitive members, part of toner forming the patch may be transferred from the intermediate transfer member to the other photosensitive members, which is referred to as re-transfer (offset).
When such re-transfer is caused, the density of the patch in the first color when it comes to a position opposing to the density detecting sensor becomes lower than that immediately after the transfer to the intermediate transfer member. The same thing can be said also with regard to a second color (for example, magenta) and a third color (for example, cyan) in greater or lesser degrees, and the density when the patch comes to the position opposing to the density detecting sensor becomes lower than that immediately after the patch is transferred from the photosensitve member to the intermediate transfer member. It is to be noted that such re-transfer is also caused in an image forming apparatus using the above-described transfer material bearing member. As a result, the densities of the toner images in the various colors formed on the respective photosensitive drums can not be appropriately controlled, and uneven image density and uneven hue or tone are caused.
It is to be noted that, conventionally, the bias to be applied to a primary transfer charger is set to be the same value both in case the patch is transferred from the photosensitive drum to the intermediate transfer member and in case the patch on the intermediate transfer member comes in contact with other downstream photosensitive drums.
Here, the cause of the above-described re-transfer is described.
As illustrated in FIG. 8A, in a transfer nip portion formed between a photosensitive drum 100 and an intermediate transfer member 101, the surface of the photosensitive drum 100 is, in this case, negatively charged, while the intermediate transfer member 101 is positively charged for the purpose of attracting toner t having negative charge. Further, in the transfer nip portion formed between the photosensitive drum 100 and the intermediate transfer member 101, there may be a case where a region A satisfying conditions for potential difference and gap exceeding a threshold of discharge.
As illustrated in FIG. 8B, when discharge is caused in the region A in the transfer nip portion, charge is exchanged, and positive charge is induced in a part of the toner in the transfer nip portion. Since the surface of the photosensitive drum 100 is negatively charged, as a result, the toner to which the positive charge has been induced on the intermediate transfer member 101 is attracted to the side of the photosensitive drum 100, which leads to the re-transfer. It is to be noted that as the contrast between the potential on the surface of the photosensitive drum 100 and the transfer voltage becomes larger, potential difference exceeding the threshold of discharge (voltage where the discharge starts) is more apt to be caused, and thus, the number of the discharge is increased and the amount of the re-transfer is increased.
FIG. 9 is the result of evaluation of the transfer efficiency and the re-transfer rate when the transfer bias is varied.
In FIG. 9, solid lines denote the transfer efficiency while dotted lines denote the re-transfer rate. Solid black dots (xe2x80xa2) plotting the transfer efficiency and the re-transfer rate denote a case where the mass per unit area of the toner (hereinafter referred to as M/S) is small (0.4 mg/cm2 on the photosensitive drum), while crosses X denote a case where M/S is large (0.8 mg/cm2 on the intermediate transfer member). It is to be noted that the transfer efficiency is the ratio of M/S on the photosensitive drum to M/S after the transfer to the intermediate transfer member in percentage, while the re-transfer rate is the ratio of M/S on the intermediate transfer member to M/S on the photosensitive drum after the intermediate transfer member comes in contact with the photosensitive drum in percentage. As the re-transfer rate becomes higher, more toner on the intermediate transfer member moves to the side of the photosensitive drum.
As is apparent from the result shown in FIG. 9, when the transfer bias satisfies the transfer efficiency of M/S=0.8 mg/cm2, the re-transfer rate is poor, while, when the re-transfer rate is satisfactory, the transfer efficiency is bad. In other words, it is thought that the above-described re-transfer is caused because no bias satisfies enough both the transfer efficiency and the re-transfer rate.
In addition, in the above-described image forming apparatus, since a conventional cleaning device dedicated for each photosensitive drum is eliminated and a xe2x80x9ccleanerless systemxe2x80x9d is adopted in which toner remaining on each photosensitive drum is collected into each developing device, when the above-described re-transfer is caused, toner in different colors is collected into the developing device and the color of the toner is mixed in the developing device. As a result, a poor image is formed in image formation thereafter.
An object of the present invention is to provide an image forming apparatus which can accurately detect an image for detection formed on an intermediate transfer member.
Another object of the present invention is to provide an image forming apparatus which can accurately detect an image for detection formed on a transfer material bearing member.
Other objects of the present invention will become apparent upon consideration of the following detailed description.